This invention relates to tomographic imaging systems such as those used for x-ray computed tomography (CT) and specifically to a tomographic system providing a gantry reference signal with adjustable spatial periodicity.
Tomographic imaging equipment produces images of transverse "slices" of an imaged object, by collecting radiation in a series of projections at various angles about the patient. The radiation may be from an extrinsic source, such as an x-ray tube positioned outside the imaged object, as in x-ray CT systems, or may be from an intrinsic source such as an ingested radioisotope, as in positron emission tomography (PET). In both cases, the data of a number of such projections is "reconstructed" to produce the slice image. The reconstruction algorithms require accurate knowledge of the "angle" at which each projection was acquired.
In a "third" or "fourth" generation CT system, for example, an x-ray source is collimated to form a fan beam with a defined fan beam angle. The fan beam is oriented to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane", and to be transmitted through an imaged object to an x-ray detector array oriented within the imaging plane.
The detector array is comprised of detector elements each of which measures the intensity of transmitted radiation along a beam projected from the x-ray source to the particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the imaged object.
The x-ray source is rotated on a gantry within the imaging plane and around the imaged object so that the angle at which the center of the fan beam intersects the imaged object may be changed. At a number of predetermined angles, termed "trigger positions", and as determined by a rotary encoder attached to the gantry, a projection is acquired comprised of the intensity signals from each of the detector elements. Together, the projections make up a projection set.
The acquired tomographic projection set is typically stored in numerical form for computer processing to later "reconstruct" a slice image according to reconstruction algorithms known in the art. The projection set may be reconstructed directly into an image by means of fan beam reconstruction techniques, or the intensity data of the constituent projections may be sorted into parallel beams and reconstructed according to parallel beam reconstruction techniques. In either case, the reconstruction algorithms require that the angle of the gantry at which each projection was acquired be accurate.
The reconstructed tomographic images may be displayed on a conventional CRT or may be converted to a film record by means of a computer controlled camera.
The quality of the tomographic image will depend in part on the number of projections acquired Often a lower resolution may be acceptable when faster scanning speed is desired For example, a commercial CT machine may take up to 7872 projections per revolution during a single eight second revolution of the gantry. At a rotational speed of one revolution every two seconds, the data acquisition chain will permit the acquisition of only 1968 projections. The resultant lower resolution image produced by these fewer samples may be acceptable in some circumstances, such as in imaging a moving organ, where the elimination of motion induced blurring is an important consideration.
The projections are equally spaced, in angle, during the rotation of the gantry. Thus, the angular separation between the projections differs for different scan speeds. For the eight second scan described above, the angular separation between each projection will be approximately 0.05 degrees whereas in the two second scan the angular separation between projections will be approximately 0.18 degrees.
The angular spacing between projections is controlled by a rotary encoder or resolver attached to the gantry and having a very fine angular resolution, such as 0.015 degrees. Typically, a counter is used to count the encoder pulses to produce a projection acquisition signal triggering each projection. For this reason, the angular spacing of the projections for all scanning speeds must be an integer multiple of this resolution.
As mentioned, the projections are taken at equal angular spacing and thus have a constant "spatial periodicity". It should be noted however, that the projections are not spaced evenly in time, that is they do not have a constant "temporal periodicity". The reason for this is that the gantry does not rotate uniformly but tends to speed up and slow down as a result of gravitational acceleration acting on the gantry's uneven distribution of mass. Also, certain techniques, such as those used to image the heart, intentionally adjust the rotational speed of the gantry during the collection of the projection set to synchronize the acquisition of projections with the beating of the heart.
While the above described technique of using a high resolution angular encoder, and dividing its output to produce the projection acquisition signal, has the advantage of being indifferent to the speed of the gantry rotation, and thus of providing a constant spatial periodicity despite variations in the encoder signal's temporal periodicity, it also has several disadvantages. First, with practicable encoder angular resolutions, the gradation in achievable spatial periodicity of the projection acquisition signals is relatively coarse. This means that certain scan rates may not be used and the operator's flexibility in trading off resolution for speed is severely limited.
Second, projection acquisition signals having spatial periodicities that are not integer multiples of the encoder's spatial periodicity are not obtainable. Dividing the encoder signal by using a counter effectively allows only integer divisors and any implicit remainder after one gantry rotation will result in the projection acquisition signals undesirably migrating in absolute angular position between gantry rotations.
Improved CT systems of the future may permit the use of greater numbers of projections in each projection set. Such improvement is likely to be incremental, however, and thus the required projection acquisition signal may not be integer multiples of the previous encoder rate. Thus, currently, upgrades of existing machines will require replacement of the entire encoder system.
Also, for certain advanced techniques such as "spot wobble", where the focal spot of the x-ray tube is shifted back and forth during rotation of the gantry, as described in co-pending application Ser. No. 07/540,995, entitled COMPUTED TOMOGRAPHY SYSTEM WITH TRANSLATABLE FOCAL SPOT, and incorporated herein by reference, it has been discovered that an optimal spatial periodicity exists dependent on the specific geometry of the system, and that in general, this spatial periodicity will not be an integer multiple of the encoder spatial periodicity. Thus, spot wobble techniques may require additional separate encoder systems.
Accordingly, a method of providing an arbitrary acquisition signal of constant spatial periodicity, independent of a particular encoder frequency would be desirable.